The mitochondrial fission-promoting protein Drp1 is linked to neuropathology in acute and chronic neurodegenerative disorders. Mdivi-1, a blood-brain barrier permeable drug thought to inhibit Drp1, protects the brain in preclinical models of stroke, cardiac arrest, and Parkinson's disease. While major efforts are underway to test whether targeting the mitochondrial fission-fusion balance in neurons can modify neurodisease, the implications of targeting mitochondrial fission in non-neuronal brain cells are almost entirely unexplored. We see a mitochondrial structural remodeling consistent with Drp1-dependent fission in microglia when they enter a proinflammatory activation state termed M1. A bioenergetic shift from oxidative phosphorylation to glycolysis promotes M1 activation, and activation is marked by production of nitric oxide (NO) and proinflammatory cytokines. Resembling the neuroinflammatory state seen in human brain injury, M1 activation can persist for months to years and result in additional pathology. A better understanding of the mechanisms contributing to M1 activation will help discover novel therapeutics that block the deleterious consequences of M1 microglial activation. We find that the Drp1 inhibitor mdivi-1 suppresses microglial NO and TNF-? production in response to lipopolysaccharide (LPS) interferon-?. We also find that mdivi-1 can inhibit reactive oxygen species (ROS) production by mitochondrial complex I in a Drp1-independent fashion. It is important to determine whether one or both of the known mdivi-1 targets, Drp1 and ROS, contribute to M1 microglial activation in vivo, and thus represent bona fide targets for therapy. This study will test the central hypothesis that in vivo, Drp1 activity but not mitochondrial ROS is required for microglial activation and the accompanying shift in cellular bioenergetics. Microglial activation will be induced in mice by intracerebroventricular administration of LPS. To investigate the role of Drp1, a novel microglia-specific inducible knockout mouse model will be developed. In addition, the existing MCAT mouse ectopically expressing matrix-targeted, H2O2-detoxifying catalase will be used to evaluate a specific requirement for mitochondrial ROS independent of other ROS. In Aim 1, we will determine whether Drp1 is necessary for the bioenergetic shift and microglial M1 activation induced by LPS in vivo. In Aim 2, we will determine whether mitochondrial ROS are required for the bioenergetic shift and microglial M1 activation induced by in vivo LPS injection. Our studies will take a crucial step toward determining whether microglial mitochondria can be targeted for therapy. Since normal Drp1-dependent fission is required for mitochondrial trafficking and quality control in neurons, a strategy specifically targeting Drp1 in microglia may ultimately be more desirable than global inhibition of mitochondrial fission. Our long-term goal is to target mitochondrial mechanisms of microglial activation in models of ischemic and traumatic brain injury, with the ultimate hope of developing a protective therapy for humans.